Composite electroconductive powder, electroconductive paste, process for producing electroconductive paste, electric circuit and process for producing electric circuit

ABSTRACT

An electroconductive paste for forming an electric circuit which is low in resistivity, high in electroconductivity and minimized in change of resistivity even after a thermal shock test and/or a humidity and DC applied test, can be obtained by using a composite electroconductive powder comprising a flake-like electroconductive powder made of a material such as silver, a silver alloy, a silver-coated copper powder or a silver-coated copper alloy powder and an unsteady-shaped electroconductive powder such as a reduced silver powder. Also, by using a composite electroconductive powder comprising an electroconductive powder having an aspect ratio of 6 or greater and an electroconductive powder having an aspect ratio of 5 or less, there can be obtained an electroconductive paste which is low in resistivity, high in electroconductivity, minimized in change of resistivity even after a thermal shock test and/or a humidity and DC applied test, and capable of improving probability of contact between the electroconductive powder particles, elevating electroconductivity of the electric circuit, and also raising electroconductivity especially when a circuit is printed on a sheet-like substrate and the printed circuit is pressed. The electric circuit formed on the surface of a substrate by using such an electroconductive paste is low in resistivity, high in electroconductivity and also excellent in migration resistance.

TECHNICAL FIELD

The present invention relates to a composite electroconductive powder,an electroconductive paste, a process for producing an electroconductivepaste, an electric circuit and a process for producing an electriccircuit.

BACKGROUND ART

As processes for forming electric circuits (wiring conductors) ofprinted wiring boards, electronic parts and such, there has generallybeen known a method which comprises coating or printing anelectroconductive paste containing silver powder with excellentconductivity as described on pages 42-46 of the October 1994 issue ofDenshi Zairyo (Electronic Materials).

Due to their good electroconductivity, the electroconductive pastescontaining silver powder have been used for forming electric circuitsand electrodes of the printed wiring boards, electronic parts and such.However, the volume resistivity (specific resistance) of the electriccircuits formed by using these electroconductive pastes usually fall inthe range of 50 to 100 μΩ·cm, and may reach only 30-40 μΩ·cm at best, sothat although it may cause few problems in case the length of theprinted circuits was small, such as several cm or less, troubles wereapt to arise when the circuit length was made 10 cm or longer due to anincrease of conductor resistance.

For obtaining a conductor with good conductor resistance, the content ofsilver power is increased, but it is hardly possible to obtain stably aspecific resistance, or resistivity, of 25 μΩ·cm or less by this means.Simple increase of the silver powder content gives rise to problems suchas upset balance with other properties such as adhesiveness.

The method comprising etching of a metal foil such as silver or copperfoil can provide a high electroconductivity and a low resistivity on theorder of several μΩ·cm, but this method has a disadvantage in that itelevates the production cost as the process is complicated. Also, theelectroconductive pastes using silver powder have a defect in that whenan electric field is applied under a high-temperature and high-humidityatmosphere, there takes place a phenomenon called migration andresulting electrodeposition of silver between the wiring conductors andelectrodes to cause short circuiting between the electrodes or thewires. For preventing such migration, several measures have been takenor are under study, such as coating the conductor surfaces with amoisture proof coating material or adding a corrosion inhibitor such asa nitrogen-containing compound to the electroconductive paste, but nosatisfactory effect has ever been obtained with these measures.

It is also suggested to use silver-palladium alloy powder in place ofsilver powder for preventing migration. However, such alloy powder iscostly as compared with silver powder, and although it is practicallyused for the small-sized wiring boards, such as those for hybrid IC, butit has not yet been practically applied to the substrates of thelarge-sized wiring boards, such as paper phenol substrate, glass epoxysubstrate and polyethylene terephthalate substrate. Use of silver-coatedcopper powder can improve the migration problem and makes it possible toobtain an inexpensive electroconductive paste, but when silver coatingis applied uniformly and thickly, no migration improving effect isproduced. Plating is an economical way for coating; for instance silverplating on an inexpensive spherical copper powder can be easilyperformed with minimized possibility of flocculation, but this methodhas a disadvantage in that the electroconductive paste using suchsilver-plated powder is increased in resistance.

An object of the present invention, therefore, is to provide a compositeelectroconductive powder which is capable of producing anelectroconductive paste for forming electric circuits which is low inresistivity, high in electroconductivity and minimized in change ofresistivity even after a thermal shock test and/or a humidity and DCapplied test.

Another object of the present invention is to provide a compositeelectroconductive powder capable of producing an electroconductive pastewhich is particularly excellent in electroconductivity and alsoexcellent in oxidation resistance and heat resistance.

Still another object of the present invention is to provide a compositeelectroconductive powder capable of producing an electroconductive pastehaving an anchoring effect when pressed.

Yet another object of the present invention is to provide a compositeelectroconductive powder capable of producing an electroconductive pastefor forming electric circuits which is low in resistivity, high inconductivity and excellent in migration resistance.

A further object of the present invention is to provide a compositeelectroconductive powder capable of producing an electroconductive pastewhich is low in resistivity, high in conductivity, minimized in changeof resistivity even after a thermal shock test and a humidity and DCapplied test, and capable of improving the probability of contactbetween the electroconductive powder particles, elevatingelectroconductivity of the electric circuits and also boostingelectroconductivity particularly when a circuit is printed on asheet-like substrate and the printed circuit is pressed.

Another object of the present invention is to provide anelectroconductive paste for forming electric circuits which is low inresistivity, high in electroconductivity and minimized in change ofresistivity even after a thermal shock test and/or a humidity and DCapplied test.

Still another object of the present invention is to provide anelectroconductive paste for forming electric circuits which is low inresistivity, capable of improving the probability of contact between theelectroconductive powder particles and elevating electroconductivity ofthe electric circuits, and also excellent in migration resistance.

Still another object of the present invention is to provide an electriccircuit which is low in resistivity, high in electroconductivity andexcellent in migration resistance.

Yet another object of the present invention is to provide an electriccircuit suited for forming fine circuits.

A further object of the present invention is to provide a process forproducing an electric circuit which is low in resistivity, high inelectroconductivity and excellent in migration resistance.

An additional object of the present invention is to provide a processfor producing an electric circuit suited for forming fine circuits.

DISCLOSURE OF INVENTION

The present invention relates to the following matters:

(1) A composite electroconductive powder comprising a flake-likeelectroconductive powder and an unsteady-shaped electroconductivepowder.

(2) A composite electroconductive powder set forth in (1) above whereinthe flake-like electroconductive powder is composed of silver, a silveralloy or a silver-coated conductor.

(3) A composite electroconductive powder set forth in (1) above whereinthe flake-like electroconductive powder is a flake-like silver-coatedconductor powder.

(4) A composite electroconductive powder set forth in (3) above whereinthe flake-like silver-coated conductor powder is a silver-coated copperpowder or a silver-coated copper alloy powder.

(5) A composite electroconductive powder set forth in (3) or (4) abovewherein the flake-like silver-coated conductor powder is a silver-coatedcopper powder with the copper powder partly exposed.

(6) A composite electroconductive powder set forth in any one of (1) to(5) above wherein the unsteady-shaped electroconductive powder iscomposed of silver or a silver alloy.

(7) A composite electroconductive powder set forth in (6) above whereinthe unsteady-shaped electroconductive powder is a reduced silver powder.

(8) A composite electroconductive powder set forth in (6) above whereinthe unsteady-shaped electroconductive powder is composed of a conductorhaving a higher hardness than silver or silver alloys and coated withsilver.

(9) A composite electroconductive powder set forth in (8) above whereinthe conductor having a higher hardness than silver or silver alloys is apowder of Co, Ni, Cr, Cu, W or an alloy thereof.

(10) A composite electroconductive powder set forth in (9) above whereinthe conductor having a higher hardness than silver or silver alloys is acopper powder or a copper alloy powder.

(11) A composite electroconductive powder set forth in any one of (8) to(10) above wherein the unsteady-shaped electroconductive powder is asilver-coated copper powder or a silver-coated copper alloy powder withthe copper or copper alloy powder partly exposed.

(12) A composite electroconductive powder comprising anelectroconductive powder having an aspect ratio of 6 or greater and anelectroconductive powder having an aspect ratio of 5 or less.

(13) A composite electroconductive powder set forth in (12) abovewherein the electroconductive powder having an aspect ratio of 6 orgreater is composed of silver, a silver alloy or a silver-coatedconductor.

(14) A composite electroconductive powder set forth in (12) abovewherein the electroconductive powder having an aspect ratio of 6 orgreater is a silver-coated conductor powder having an aspect ratio of 6or greater.

(15) A composite electroconductive powder set forth in (14) abovewherein the silver-coated conductor powder having an aspect ratio of 6or greater is a silver-coated copper powder or a silver-coated copperalloy powder.

(16) A composite electroconductive powder set forth in (14) or (15)above wherein the silver-coated conductor powder having an aspect ratioof 6 or greater is a silver-coated copper powder or a silver-coatedcopper alloy powder with the copper or copper alloy powder partlyexposed.

(17) A composite electroconductive powder set forth in any one of (12)to (16) above wherein the electroconductive powder having an aspectratio of 5 or less is composed of silver or a silver alloy.

(18) A composite electroconductive powder set forth in (17) abovewherein the electroconductive powder having an aspect ratio of 5 or lessis a reduced silver powder.

(19) A composite electroconductive powder set forth in (17) abovewherein the electroconductive powder having an aspect ratio of 5 or lessis a conductor having a higher hardness than silver or silver alloys andcoated with silver.

(20) A composite electroconductive powder set forth in (19) abovewherein the conductor having a higher hardness than silver or silveralloys is a powder of Co, Ni, Cr, Cu, W or an alloy thereof.

(21) A composite electroconductive powder set forth in (19) abovewherein the conductor having a higher hardness than silver or silveralloys is a copper powder or a copper alloy powder.

(22) A composite electroconductive powder set forth in any one of (19)to (21) above wherein the unsteady-shaped electroconductive powder is asilver-coated copper powder with the copper powder partly exposed.

(23) An electroconductive paste comprising a composite electroconductivepowder set forth in any one of (1) to (22) above, a binder and asolvent.

(24) An electroconductive paste set forth in (23) above wherein thecomposite electroconductive powder is contained in an amount of 85-93%by weight based on the solids of the electroconductive paste.

(25) An electroconductive paste set forth in (23) or (24) abovecomprising a composite electroconductive powder set forth in any one of(1) to (11) above, a binder and a solvent, wherein the unsteady-shapedelectroconductive powder is contained in an amount of 5-50% by weight asagainst 95-50% by weight of the flake-like electroconductive powder.

(26) An electroconductive paste set forth in (23) or (24) abovecomprising a composite electroconductive powder set forth in any one of(12) to (22) above, a binder and a solvent, wherein theelectroconductive powder having an aspect ratio of 5 or less iscontained in an amount of 5-50% by weight as against 95-50% by weight ofthe electroconductive powder having an aspect ratio of 6 or greater.

(27) An electric circuit formed on the surface of a substrate made byusing an electroconductive paste set forth in any one of (23) to (26)above.

(28) An electric circuit set forth in (27) above wherein the resistivityof the electric circuit formed on the substrate surface is 25 μΩ·cm orless.

(29) A process for making an electric circuit which comprises forming acircuit pattern with an electroconductive paste set forth in any one of(23) to (26) above on the surface of a substrate, and then pressing andcuring the paste.

(30) A process for making an electric circuit set forth in (29) abovewherein the resistivity of the electric circuit formed on the substratesurface is 25 μΩ·cm or less.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plane view showing a state where a silver conductor circuithas been printed on a polyethylene terephthalate film. Reference numeral1 designates a silver conductor circuit, and 2 a polyethyleneterephthalate film.

FIG. 2 is a plane view of a circuit board in an embodiment of thepresent invention. Reference numeral 3 indicates a substrate, 4 acircuit, 5 a chipmounting portion, and 6 a circuit board.

FIG. 3 is a plane view of an IC chip. Reference numeral 7 indicates atest circuit, 8 an IC chip, and 9 a silicon substrate.

FIG. 4 is a schematic illustration showing a state where an IC chip hasbeen mounted on a chipmounting portion of a circuit board.

FIG. 5 is a plane view showing a state where a silver electroconductivecircuit has been printed in the pattern of a comb on a polyethyleneterephthalate film.

BEST MODE FOR CARRYING OUT THE INVENTION

In the present invention, in the case of a combination of a flake-likeelectroconductive powder and an unsteady-shaped electroconductive powderor a combination of an electroconductive powder having an aspect ratioof 6 or greater and an electroconductive powder having an aspect ratioof 5 or less, the probability of contact between the electroconductivepowders can be improved and electroconductivity of the electric circuitis elevated, especially in case the circuit is printed on a sheet-likesubstrate and the printed circuit is pressed.

The "aspect ratio" of an electroconductive powder referred to in thepresent invention means the ratio of the major diameter to the minordiameter (major diameter/minor diameter) of the electroconductive powderparticles. In the present invention, the electroconductive powderparticles are mixed well in a curable resin of a low viscosity, themixture is allowed to stand, letting the powder particles settle downwhile allowing the resin to cure, and the obtained cured product is cutin the vertical direction. The shapes of the particles appearing on thecut section are observed under an electron microscope, and the majordiameter/minor diameter ratio of every one of at least 100 randomlypicked up particles is determined. The mean value of the determinationsis given as aspect ratio.

Here, the "minor diameter" is the distance between the twoparticle-holding parallel lines which are the smallest in spacing in thecombinations of the two parallel lines which hold therebetween theparticles appearing on said cut section, with each line contacting theoutside of a particle. On the other hand, the "major diameter" is thedistance between the two particle-holding parallel lines which are thelargest in spacing in the combinations of the two parallel lines holdingthe particles therebetween and contacting the outsides of the particles,these parallel lines being orthogonal to those which determine the minordiameter mentioned above. The rectangle defined by these four linesprovides a space in which a particle just fits.

The concrete method employed in the present invention will be describedlater.

The "flake-like electroconductive powder" refers to an electroconductivepowder comprising fine particles which are substantially flaky in shape,for instance, a flaky electroconductive powder. The "unsteady-shapedelectroconductive powder" refers to electroconductive powders of variousshapes other than flake-like, which include, for example, spherical,cubic, tetrahedral, briquet or semi-spherical powders, the powdershaving projections on the surface like confetti, and mixtures of thesepowders. An example of the mixed electroconductive powders containingthe powders of various shapes is a reduced silver powder.

An electroconductive powder having an aspect ratio of 6 or greater andthe one having an aspect ratio of 5 or less can be used as theflake-like electroconductive powder and the unsteady-shapedelectroconductive powder, respectively.

The flake-like electroconductive powders are, in many cases, suited foruse as an electroconductive powder having an aspect ratio of 6 orgreater. This type of electroconductive powder also includes so-calleddendrite powder, and this powder may be used in combination withflake-like electroconductive powders. As the electroconductive powderhaving an aspect ratio of 6 or greater, it is desirable to use a powderhaving an aspect ratio of 7 or greater, more preferably 8 or greater,even more preferably 10 or greater, as a high-electroconductivity pastecan be obtained with these powders. Thus, when considered from bothfacets of shape and aspect ratio, it is desirable to use a flake-likeelectroconductive powder having an aspect ratio of 7 or greater, morepreferably a flake-like electroconductive powder having an aspect ratioof 8 or greater, most preferably a flake-like electroconductive powderhaving an aspect ratio of 10 or greater, in view of such matters as highelectroconductivity and viscosity of the electroconductive paste.

The average diameter of the particles of the flake-likeelectroconductive powder or the powder having an aspect ratio of 6 orgreater is preferably 25 μm or less, more preferably 20 μm or less, evenmore preferably 10 μm or less, from the viewpoint of retainedprintability. The average particle diameter referred to herein can bemeasured by a laser scattering type particle size distribution meter. Inthe present invention, Master-Sizer (mfd. by Malvern Instruments, Ltd.)was used for measuring the average particle diameter.

Most of said unsteady-shaped electroconductive powders are applicable asthe electroconductive powder having an aspect ratio of 5 or less. As theelectroconductive powder with an aspect ratio of 5 or less, it isdesirable that the aspect ratio is 4 or less, more preferably 3 or less,even more preferably 2.5 or less, as a high-electroconductivity pastecan be obtained.

The average particle diameter of the unsteady-shaped electroconductivepowder or the powder having an aspect ratio of 5 or less is preferablyin the range of 3-20 μm, more preferably in the range of 3-10 μm, forthe reason of better printability. The average particle diameterreferred to herein can be measured by a laser scattering type particlesize distribution meter as in the above case. In the present invention,Master-Sizer (mfd. by Malvern Instruments Ltd.) was used for themeasurement.

As the material of the electroconductive powders, silver or a silveralloy is preferred from the viewpoint of conductivity and oxidationresistance.

As said silver alloy, it is preferable to use an alloy with palladium(for example, about 1-5 wt % in the silver alloy) or platinum (forexample, about 1 wt % in the silver alloy).

Reduction-in-liquid phase technique is known as one of the methods formaking said silver powder. Since the silver powder produced by thismethod is a fine powder of several μm in average particle diameter, thismethod is widely utilized as an industrial powder production method.This reduction-in-liquid phase technique is a method in which silver isdissolved with an acid, then neutralized with an alkali and reduced inliquid by adding a reducing agent such as formalin or starch, followedby pulverization. The powder obtained by this method is called reducedsilver powder, and its shape, although close to lump, is not constantbut irregular. This reduced silver powder can be used in the presentinvention as an unsteady-shaped electroconductive powder or anelectroconductive powder having an aspect ratio of 5 or less.

As the electroconductive powders, there can also be used silver-coatedelectroconductive powders produced by coating an electroconductivematerial, which is not silver or a silver alloy, with silver or a silveralloy.

Such silver-coated electroconductive powders can also be used as theunsteady-shaped electroconductive powder, but in this case, theelectroconductive material to be coated is preferably the one having ahigher hardness than silver or silver alloys. As such electroconductivematerial, there can be used such metals as Co, Ni, Cr, Cu and W oralloys thereof in powdery form, of which copper powder or copper alloypowder is preferred.

Use of such an electroconductor powder is desirable because when theformed electric circuit is pressed, the unsteady-shapedelectroconductive powder is allowed to get into the flake-like silver orsilver alloy powder to elevate electroconductivity of the electriccircuit.

As said copper alloy powder, there can be used, for instance, a copperand tin alloy powder or a copper and zinc alloy powder.

Such methods as substitution plating, electroplating and electrolessplating are available for coating the particle surfaces of theunsteady-shaped electroconductive powder or the powder having an aspectratio of 5 or less with silver, but substitution plating is preferredbecause of high adhesion between the unsteady-shaped electroconductivepowder or the powder having an aspect ratio of 5 or less and silver andlow running cost. The coating weight of silver on the particle surfacesof the unsteady-shaped electroconductive powder or the powder having anaspect ratio of 5 or less is preferably in the range of 3-50% by weight,more preferably 3-20% by weight, based on the unsteady-shapedelectroconductive powder or the powder having an aspect ratio of 5 orless, in view of cost, electrolytic corrosion inhibitory effect andother factors.

Use of any of said silver-coated conductor powders is desirable becauseof excellent migration resistance. It is possible to use thesilver-coated conductor powder with the coated conductor partly exposed(hereinafter referred to as exposed coated conductor powder).

The exposed coated conductor powder can be used for both of theflake-like conductor powder or the electroconductive powder having anaspect ratio of 6 or greater and the unsteady-shaped electroconductivepowder or the electroconductive powder having an aspect ratio of 5 orless.

The exposed area of the electroconductive powder is preferably 50% orless, more preferably 20% or less, for obtaining goodelectroconductivity.

The spherical silver-coated copper powder or silver-coated copper alloypowder which has been subjected to substitution plating tends toincrease in resistance because of limited contact points. It istherefore recommended to give impact to the substitution platedspherical silver-coated copper powder or silver-coated copper alloypowder to change the shape of the powder particles into a flake-likeform or a form having an aspect ratio of 6 or greater. Specifically,such change of the powder particle shape can be effected by such amethod as ball milling or vibration milling.

As for the blending ratios of the flake-like electroconductive powderand the unsteady-shaped electroconductive powder or theelectroconductive powder having an aspect ratio of 6 or greater and theelectroconductive powder having an aspect ratio of 5 or less, it isdesirable for elevating electroconductivity that the ratio of theunsteady-shaped electroconductive powder or the electroconductive powderhaving an aspect ratio of 5 or less is in the range of 5-50% by weightto 95-50% by weight of the flake-like electroconductive powder or theelectroconductive powder having an aspect ratio of 6 or greater. It ismore desirable when the ratio of the unsteady-shaped electroconductivepowder or the electroconductive powder having an aspect ratio of 5 orless is in the range of 20-40% by weight to 80-60% by weight of theflake-like electroconductive powder or the electroconductive powderhaving an aspect ratio of 6 or greater.

The electroconductive paste contains a composite electroconductivepowder comprising said types of electroconductive powder, a binder and asolvent. In this electroconductive paste, the content of the compositeelectroconductive powder is preferably 85-93% by weight, more preferably87-90% by weight, in view of conductor resistance to the solid matter ofthe electroconductive paste, economy and adhesion.

As the binder, organic adhesive materials such as liquid epoxy resins,phenol resins, unsaturated polyester resins and the like can be used. Asthe solvent, terpineol, ethyl carbitol, carbitol acetate, butylCellosolve and the like can be used. The electroconductive paste can beobtained by adding, beside said materials, a curing agent of the organicadhesive materials, such as 2-ethylmethylimidazole, and if necessary acorrosion inhibitor such as benzothiazole or benzoimidazole, finegraphite powder, etc., and mixing them uniformly. The contents of thebinder and the solvent are preferably in the ranges of 7-15% by weightfor the binder and 10-35% by weight for the solvent, more preferably inthe ranges of 7-12% by weight for the binder and 15-25% by weight forthe solvent, in view of electroconductivity, adhesion and printability.The content of the curing agent is preferably in the range of 0.5-10parts by weight, more preferably 1-8 parts by weight, per 100 parts byweight of the binder in view of workability. A corrosion inhibitor andfine graphite powder may be added as desired. When they are added, thecontent of the corrosion inhibitor is preferably in the range of 0.1-isparts by weight per 100 parts by weight of the binder, and the contentof the fine graphite powder is preferably in the range of 1-10% byweight based on the electroconductive paste.

The method for forming the electric circuits is not subject to anyspecific restrictions; the circuits can be formed by known methods, forexample, screen printing the electroconductive paste or using acomputer-controlled drawing machine.

As the substrate, a polyethylene terephthalate film, a polyimide film, apolyamide-imide film, a paper/phenol laminate, an epoxy/glass laminate,a polyimide/glass substrate and the like can be used.

In the present invention, the resistivity of the electric circuit ispreferably 25 μΩ·cm or less, more preferably 15 μΩ·cm or less. When theresistivity exceeds 25 μΩ·cm, electroconductivity tends to lowercorrespondingly, which may cause an excessive drop of voltage of theelectric circuit, making it difficult to form a fine electric circuit.It is especially desirable that the resistivity of the electric circuitbe less than 10 μΩ·cm, because in this case it is possible to use thefine and long lines such as coiled plane aerial for the electriccircuit. For reducing the resistivity of the electric circuit to lessthan 25 μΩ·cm, the circuit pattern formed with said electroconductivepaste on the substrate surface is pressed to densify the circuitpattern. For pressing, there can be employed a suitable means, such asapplying pressure by using a platen or pressing with the rolls, which iscapable elevating the contacting efficiency of the powder particles inthe conductive layer formed with said electroconductive paste. It isdesirable that the binder in the electroconductive layer is in asoftened state when pressing is performed. If the binder is in ahalf-cured or cured state, it is preferably softened by heating beforepressing. The binder may be cured after or during pressing.

EXAMPLES

The examples of the present invention are illustrated below.

Example 1

60 parts by weight of a bisphenol A epoxy resin (trade name Epikote 834,produced by Yuka Shell Epoxy Co., Ltd.) and 40 parts by weight of abisphenol A epoxy resin (trade name Epikote 828, produced by Yuka ShellEpoxy Co., Ltd.) were dissolved by heating and then cooled to roomtemperature, after which 5 parts by weight of 2-ethyl-4-methylimidazole(produced by Shikoku Chemicals Corp.), 20 parts by weight of ethylcarbitol and 20 parts by weight of butyl Cellosolve were added anduniformly mixed to prepare a resin composition.

Then 210 parts by weight (79.2% by weight) of a flaky silver powderhaving an aspect ratio of 8 and an average major particle diameter of 8μm (trade name TCG-1, produced by Tokuriki Chemical Research Co., Ltd.)and 55 parts by weight (20.8% by weight) of a silver powder having anaspect ratio of 2.3 and an average major particle diameter of 7 μm(produced by Rare Metallic Co., Ltd.) (hereinafter referred to as silverpowder having an aspect ratio of 2.3) were blended and this blend wasadded to 145 parts by weight of the previously prepared resincomposition and uniformly mixed and dispersed by a mixing and grindingmachine and a threeroll mill to obtain an electroconductive paste. Thecontent of the composite silver powder comprising of the flaky reducedsilver powder and the silver powder having an aspect ratio of 2.3 was80% by weight based on the solids of the electroconductive paste.

Then, by using the above electroconductive paste, a silver conductorcircuit 1 shown in FIG. 1 was printed on a 125 μm thick polyethyleneterephthalate film and subjected to a heat treatment at 60° C. for 30minutes and then at 145° C. for 30 minutes in the atmosphere to obtain awiring board. Reference numeral 2 in FIG. 1 indicates a polyethyleneterephthalate film.

The resistivity of the obtained wiring board was measured, finding itwas 52 μΩ·cm. As a result of a thermal shock test of this wiring board,the rate of change of resistivity was 5%. According to the result of ahumidity and DC applied test of this wiring board, the rate of change ofresistivity was 8%. The thermal shock test was conducted by carrying out100 cycles of heating at 125° C. for 30 minutes and cooling at -65° C.for 30 minutes, and the humidity and DC applied test was conducted bymaintaining the test piece in the atmosphere of 40° C. and 90% RH for1,000 hours.

The concrete method of measuring the aspect ratio in the instant Exampleis shown below. 8 g of the base (No. 20-8130) comprising a low-viscosityepoxy resin (produced by Buehler Ltd.) and 2 g of a curing agent (No.20-8132) were mixed, in which 2 g of an electroconductive powder wasmixed and dispersed well, and the mixture was defoamed in vacuo at 30°C. and then allowed to stand at 30° C. for 6-8 hours, causing theparticles to be sedimented and cured. Thereafter, the obtained curedproduct was cut in the vertical direction, and by magnifying the cutsection 2,000 times by an electron microscope, the major diameter/minordiameter ratio of the 100 particles appearing on the cut section wasmeasured, with the average value of the measurements being given asaspect ratio.

Example 2

An electroconductive paste was obtained by following the same process asin Example 1 except that 400 parts by weight (80% by weight) of theflaky silver powder and 100 parts by weight (20% by weight) of thesilver powder having an aspect ratio of 2.3 were blended. The content ofthe composite silver powder comprising the flaky reduced silver powderand the silver powder having an aspect ratio of 2.3 was 85% by weightbased on the solids of the electroconductive paste. Thereafter the sameprocedure as in Example 1 was followed to make a wiring board and itscharacteristics were evaluated. As a result, resistivity of the wiringboard was 43 μΩ·cm. Also, as a result of the thermal shock test of thiswiring board, the rate of change of resistivity was 4%. The result ofthe humidity and DC applied test showed the rate of change ofresistivity was 7%.

Example 3

An electroconductive paste was obtained by following the same process asin Example 1 except that 320 parts by weight (80% by weight) of flakysilver powder and 80 parts by weight (20% by weight) of silver powderhaving an aspect ratio of 2.3 were blended. The content of the compositesilver powder comprising the flaky reduced silver powder and the silverpowder having an aspect ratio of 2.3 was 86% by weight. A wiring boardwas manufactured by following the same procedure as in Example 1, andits characteristics were evaluated. As a result, resistivity of thewiring board was 39 μΩ·cm. The result of the thermal shock test of saidwiring board showed that the rate of change of resistivity was 5%.According to the result of the humidity and DC applied test, the rate ofchange of resistivity was 6%.

Example 4

The wiring board obtained in Example 3 was hot pressed under theconditions of 100° C. and 10 MPa to densify the silver conductorcircuit, and its characteristics were evaluated. As a result,resistivity of the densified wiring board was 22 μΩ·cm. The result ofthe thermal shock test of the densified wiring board showed that therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 4%.

Comparative Example 1

400 parts by weight of the flaky reduced silver powder used in Example 1was added to 145 parts by weight of the resin composition obtained inExample 1 and uniformly mixed and dispersed in the same way as inExample 1 to obtain an electroconductive paste. Thereafter a wiringboard was manufactured by following the same process as in Example 1,and its characteristics were evaluated. As a result, resistivity of thewiring board was 62 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 10%. The result ofthe humidity and DC applied test showed that the rate of change ofresistivity was 9%.

Comparative Example 2

A wiring board was obtained by following the same process as in Example1 except for use of the electroconductive paste obtained in ComparativeExample 1. Then said wiring board was hot pressed under the sameconditions as in Example 4 to density the silver conductor circuits, andits characteristics were evaluated. As a result, resistivity of thedensified wiring board was 58 μΩ·cm. As a result of the thermal shocktest of the densified wiring board, the rate of change of resistivitywas 10%. According to the result of the humidity and DC applied test,the rate of change of resistivity was 9%.

Example 5

An Ni powder having an aspect ratio of 2 and an average major particlediameter of 3 μm (produced by High Purity Chemicals Co., Ltd.) wasdegreased by an acidic cleaner (trade name L-5B, produced by NipponMacdermid Co., Ltd.), washed with water, subjected to electrolessplating in a mixed bath of 20 g AgCN/l liter H₂ O and 40 g NaCN/l litreH₂ O so that the amount of silver became 15% by weight based on said Nipowder, then washed with water and dried to obtain a silver-coated Nipowder.

Then 210 parts by weight (91.3% by weight) of a flaky silver powderhaving an aspect ratio of 8 and an average major particle diameter of 8μm (trade name TCG-1 produced by Tokuriki Chemical Research Co., Ltd.)and 20 parts by weight (8.7% by weight) of the above silver-coated Nipowder were blended, and the blend was added to 145 parts by weight ofthe resin composition obtained in Example 1 and uniformly mixed anddispersed by a mixing and grinding machine and a three-roll mill toobtain an electroconductive paste. The content of the compositeelectroconductive powder comprising the flaky silver powder and thesilver-coated Ni powder was 61.3% by weight based on the solids of theelectroconductive paste.

A wiring board was manufactured by following the same procedure as inExample 1 and its characteristics were evaluated. As a result,resistivity of the wiring board was 43 μΩ·cm. As a result of the thermalshock test of the wiring board, the rate of change of resistivity was5%. The result of the humidity and DC applied test gave 8% as the rateof change of resistivity.

Example 6

An electroconductive paste was obtained by following the same process asin Example 5 except that 400 parts by weight (88.9% by weight) of theflaky silver powder used in Example 5 and 50 parts by weight (11.1% byweight) of the silver-coated Ni powder obtained in Example 5 wereblended. The content of the composite electroconductive powdercomprising the flaky silver powder and the silver-coated Ni powder was70.7% by weight based on the solids of the electroconductive paste. Awiring board was made by following the same process as in Example 5 andits characteristics were evaluated. As a result, resistivity of thewiring board was 43 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 4%. According to theresult of the humidity and DC applied test, the rate of change ofresistivity was 7%.

Example 7

An electroconductive paste was obtained by following the same process asin Example 5 except that 320 parts by weight (91.4% by weight) of theflaky silver powder used in Example 5 and 30 parts by weight (8.6% byweight) of the silver-coated Ni powder obtained in Example 5 wereblended. The content of the composite electroconductive powdercomprising the flaky silver powder and the silver-coated Ni powder was70.7% by weight based on the solids of the electroconductive paste. Awiring board was manufactured by the following the same process as inExample 5 and its characteristics were evaluated. As a result,resistivity of the wiring board was 38 μΩ·cm. As a result of the thermalshock test of said wiring board, the rate of change of resistivity was5%. According to the result of the humidity and DC applied test, therate of change of resistivity was 6%.

Example 8

An electroconductive paste was obtained by following the same process asin Example 5 except that 320 parts by weight (80.0% by weight) of theflaky silver powder used in Example 5 and 80 parts by weight (20.0% byweight) of the silver-coated Ni powder were blended. The content of thecomposite electroconductive powder comprising the flaky silver powderand the silver-coated Ni powder was 89.9% by weight based on the solidsof the electroconductive paste. Then said wiring board was hot pressedunder the conditions of 100° C. and 10 MPa to densify the printedcircuit, and its characteristics were evaluated. As a result,resistivity of the densified wiring board was 20 μΩ·cm. As a result ofthe thermal shock test of the densified wiring board, the rate of changeof resistivity was 5%. According to the result of the humidity and DCapplied test, the rate of change of resistivity was 4%.

Example 9

400 parts by weight (66.7% by weight) of a flaky silver powder B havingan aspect ratio of 11 and an average major diameter of 20 μm, obtainedby grinding a silver powder A having an aspect ratio of 8 and an averagemajor particle diameter of 8 μm (trade name TCG-1 produced by TokurikiChemical Research Co., Ltd.) by a mixing and grinding machine (mfd. byMitsui Mining Co., Ltd.), and 200 parts by weight (33.3% by weight) of asilver powder having an aspect ratio of 2.3 and an average particlediameter of 7 μm (produced by Rare Metallic Co., Ltd.) were blended, andthis blend was added to 145 parts by weight of the resin compositionobtained in Example 1, and uniformly mixed and dispersed by a mixing andgrinding machine and a three-roll mill to obtain an electroconductivepaste. The ratio of the flaky silver powder B to the silver powderhaving an aspect ratio of 2.3 was 6.67:3.33 by volume. The compositeelectroconductive (silver) powder was contained in an amount of 85.1%based on the solids of the electroconductive paste.

The above electroconductive paste was printed to a test pattern shown inFIG. 2 and cured by heating to form a 25 μm thick circuit 4 to obtain acircuit board 6. A polyethylene terephthalate film (125 μm thick) wasused as the substrate 3, and curing by heating was conducted under theconditions of 60° C./30 minutes and 145° C./45 minutes. Resistivity ofthe obtained circuit board 6 was 42 μΩ·cm. As a result of the thermalshock test of said circuit board 6, the rate of change of resistivitywas 5%. According to the result of the humidity and DC applied test, therate of change of resistivity was 8%. The thermal shock test wasconducted by carrying out 100 cycles of heating at 125° C. for 30minutes and cooling at -65° C. for 30 minutes, and the humidity and DCapplied test was conducted by keeping the test piece in the atmosphereof 40° C. and 90% RH for 1,000 hours. On the other hand, an IC chip 8made by forming a test circuit 7 on a silicon substrate 9 shown in FIG.is was placed on the upper side of the chip mounting portion of saidcircuit board 6 via an anisotropic conductive sheet (trade name Anisorum8201, produced by Hitachi Chemical Co., Ltd.) (not shown) so that thetest circuit 7 came on the underside as shown in FIG. 4 and connected.The average value of connecting resistance including the anisotropicelectroconductive sheet was 45 mΩ, the maximum value thereof being 63mΩ.

Example 10

A blend of 350 parts by weight (50% by weight) of the flaky silverpowder B used in Example 9 and 350 parts by weight (50% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition obtained in Example 1 and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 5:5 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 43.2% by volume (86.9% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9 and its characteristics were evaluated. As aresult, resistivity was 41 μΩ·cm. As a result of the thermal shock testof said circuit board, the rate of change of resistivity was 4%.According to the result of the humidity and DC applied test, the rate ofchange of resistivity was 7%. When a test circuit was connected by usingan anisotropic electroconductive sheet in the same way as in Example 9,the average value of connecting resistance was 40 mΩ, the maximum valuethereof being 55 mΩ.

Example 11

A blend of 650 parts by weight (72.2% by weight) of the flaky silverpowder used in Example 9 and 250 parts by weight (27.8% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition obtained in Example 1, and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 7.22:2.78 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 49.5% by volume (89.6% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9, and its characteristics were evaluated. As aresult, resistivity was 39 μΩ·cm. As a result of the thermal shock testof said circuit board, the rate of change of resistivity was 5%.According to the result of the humidity and DC applied test, the rate ofchange of resistivity was 6%. The average value of connecting resistancemeasured after connecting a test circuit by using an anisotropicelectroconductive sheet in the same way as in Example 9 was 38 mΩ, themaximum value thereof being 58 mΩ.

Example 12

A blend of 700 parts by weight (58.3% by weight) of the flaky silverpowder B used in Example 9 and 500 parts by weight (41.7% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition used in Example 1 and uniformly mixedand dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 5.83:4.17 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 56.6% by volume (91.9% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9 except that heat curing was conducted under theconditions of 60° C./30 minutes and 110° C./one hour. Then the circuitboard was hot pressed under the conditions of 100° C. and 9.8 MPa todensify the circuit, and the characteristics of the board wereevaluated. As a result, resistivity (of the circuit board) was 9.1μΩ·cm. As a result of the thermal shock test of said circuit board, therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 4%.The average value of connecting resistance measured after connecting thecircuit by using an anisotropic conductive sheet in the same way as inExample 9 was 32 mΩ, the maximum value being 42 mΩ.

Comparative Example 3

600 parts by weight of the flaky silver powder B used in Example 9 wasadded to 145 parts by weight of the resin composition obtained inExample 1, and then uniformly mixed and dispersed in the same way as inExample 9 to obtain an electroconductive paste. A circuit board was madeby following the same process as in Example 9 and its characteristicswere evaluated. As a result, resistivity (of the circuit board) was 46μΩ·cm. The result of the thermal shock test of said circuit board showedthat the rate of change of resistivity was 10%. According to the resultof the humidity and DC applied test, the rate of change of resistivitywas 9%. The average value of connecting resistance measured afterconnecting the circuit by using an anisotropic conductive sheet as inExample 9 was 1.05 mΩ, the maximum value being 12 ma.

Example 13

A blend of 380 parts by weight (63.3% by weight) of the flaky silverpowder B used in Example 9 and 220 parts by weight (36.7% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition obtained in Example 1, and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 6.33:3.67 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 39.5% by volume (85.1% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9, and its characteristics were evaluated. As aresult, resistivity (of the circuit board) was 43 μΩ·cm. As a result ofthe thermal shock test of said circuit board, the rate of change ofresistivity was 5%. The result of the humidity and DC applied gave 8% asthe rate of change of resistivity. The average value of connectingresistance measured after connecting the test circuit by using ananisotropic electroconductive sheet in the same way as in Example 9 was42 mΩ, the maximum value being 60 mΩ.

Example 14

A blend of 380 parts by weight (54.2% by weight) of the flaky silverpowder B used in Example 9 and 320 parts by weight (45.8% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition obtained in Example 1, and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 5.42:4.58 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 43.2% by volume (86.9% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9, and its characteristics were evaluated. As aresult, resistivity (of the circuit board) was 43 μΩ·cm. As a result ofthe thermal shock test of said circuit board, the rate of change ofresistivity was 4%. According to the result of the humidity and DCapplied test, the rate of change of resistivity was 7%. The averagevalue of connecting resistance measured after connecting the testcircuit by using an anisotropic electroconductive sheet in the same wayas in Example 9 was 38 mΩ, the maximum value being 52 mΩ.

Example 15

A blend of 600 parts by weight (66.7% by weight) of the flaky silverpowder B used in Example 9 and 300 parts by weight (33.3% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition obtained in Example 1, and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 6.67:3.33 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 49.5% by volume (89.6% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9, and its characteristics were evaluated. As aresult, resistivity (of the circuit board) was 35 μΩ·cm. As a result ofthe thermal shock test of said circuit board, the rate of change ofresistivity was 5%. According to the result of the humidity and DCapplied test, the rate of change of resistivity was 6%. The averagevalue of connecting resistance measured after connecting the testcircuit by using an anisotropic electroconductive sheet in the same wayas in Example 9 was 36 mΩ, the maximum value being 56 mΩ.

Example 16

A blend of 650 parts by weight (54.2% by weight) of the flaky silverpowder B used in Example 9 and 550 parts by weight (45.8% by weight) ofthe silver powder having an aspect ratio of 2.3 was added to 145 partsby weight of the resin composition used in Example 1, and uniformlymixed and dispersed in the same way as in Example 9 to obtain anelectroconductive paste. The ratio of the flaky silver powder B to thesilver powder having an aspect ratio of 2.3 was 5.42:4.58 by volume. Thecomposite electroconductive (silver) powder was contained in an amountof 56.6% by volume (91.9% by weight) based on the solids of theelectroconductive paste. A circuit board was made by following the sameprocess as in Example 9 and hot pressed under the conditions of 100° C.and 9.8 MPa to densify the circuits, and its characteristics wereevaluated. As a result, resistivity (of the circuit board) was 8.7μΩ·cm. As a result of the thermal shock test of said circuit board, therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 4%.The average value of connecting resistance measured after connecting thecircuit by using an anisotropic electroconductive sheet in the same wayas in Example 9 was 30 mΩ, the maximum value being 40 mΩ.

Example 17

A blend of 450 parts by weight (75% by weight) of the flaky silverpowder having as aspect ratio of 8 and an average major particlediameter of 8 μm (trade name TCG-1 produced by Tokuriki ChemicalResearch Co., Ltd.) and 150 parts by weight (25% by weight) of thesilver powder having an aspect ratio of 2.3 and an average majorparticle diameter of 7 μm (produced by Rare Metallic Co., Ltd.)(hereinafter referred to as silver powder having an aspect ratio of 2.3)was added to 145 parts by weight of the resin composition obtained inExample 1 and uniformly mixed and dispersed by a mixing and grindingmachine and a three-roll mill to obtain an electroconductive paste. Thecontent of the flaky silver powder and the silver powder having anaspect ratio of 2.3 was 39.5% by volume (85.1% by weight) based on thesolids of the electroconductive paste.

A wiring board was manufactured by following the same process as inExample 1, and this wiring board was hot pressed under the conditions of110° C. and 10 MPa for 2 minutes and then cured at 145° C. for 30minutes to form an electric circuit with a densified circuit pattern.Resistivity of this electric circuit was 13 μΩ·cm. As a result of thethermal shock test of said wiring board, the rate of change ofresistivity was 5%. According to the result of the humidity and DCapplied test of said wiring board, the rate of change of resistivity was7%. The thermal shock test was conducted by carrying out 100 cycles ofheating at 125° C. for 30 minutes and cooling at -65° C. for 30 minutes,and the humidity and DC applied test was conducted by maintaining thetest piece in an atmosphere of 40° C. and 90% RH for 1,000 hours.

Example 18

An electroconductive paste was obtained by following the same process asin Example 17 except that 550 parts by weight (84.6% by weight) of theflaky silver powder used in Example 17 and 100 parts by weight (15.4% byweight) of the silver powder having an aspect ratio of 2.3 were blended.The content of the flaky silver powder and the silver powder having anaspect ratio of 2.3 was 41.4% by volume (86.1% by weight) based on thesolids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon, and its characteristics were evaluated. As a result,resistivity of the electric circuit was 10 μΩ·cm. As a result of thethermal shock test of said wiring board, the rate of change ofresistivity was 4%. The result of the humidity and DC applied testshowed 7% as the rate of change of resistivity.

Example 19

An electroconductive paste was obtained by following the same process asin Example 17 except that 900 parts by weight (81.8% by weight) of theflaky silver powder used in Example 17 and 200 parts by weight (18.2% byweight) of the silver powder having an aspect ratio of 2.3 were blended.The content of the flaky silver powder and the silver powder having anaspect ratio of 2.3 was 55% by volume (91.5% by weight) based on thesolids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon, and its characteristics were evaluated. As a result,resistivity of the electric circuit was 8.2 μΩ·cm. As a result of thethermal shock test of said wiring board, the rate of change ofresistivity was 5%. According to the result of the humidity and DCapplied test, the rate of change of resistivity was 6%.

Example 20

The wiring board obtained in Example 19 was hot pressed by heated rollsunder the conditions of 125° C. and 980 N/cm at a rate of 30 cm/min andcured at 145° C. for 30 minutes to form an electric circuit with adensified circuit pattern. Resistivity of this electric circuit was 8.3μΩ·cm. As a result of the thermal shock test of said wiring board, therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 4%.

Example 21

An electroconductive paste was obtained by following the same process asin Example 17 except that 410 parts by weight (66.7% by weight) of theflaky silver powder used in Example 17 and 205 parts by weight (33.3% byweight) of the silver powder having an aspect ratio of 2.3 were blended.The content of the flaky silver powder and the silver powder having anaspect ratio of 2.3 was 40% by volume (85.4% by weight) based on thesolids of the electroconductive paste. A wiring board was obtained byfollowing the same process as in Example 17.

Then the above wiring board was hot pressed under the conditions of 100°C. and 5 MPa for 2 minutes and then cured at 145° C. for 30 minutes toform an electric circuit with a densified circuit pattern. Resistivityof this electric circuit was 12 μΩ·cm. As a result of the thermal shocktest of said wiring board, the rate of change of resistivity was 5%.According to the result of the humidity and DC applied test, the rate ofchange of resistivity was 8%. The thermal shock test was conducted bycarrying out 100 cycles of heating at 125° C. for 30 minutes and coolingat -65° C. for 30 minutes, and the humidity and DC applied test wasconducted by maintaining the test piece in an atmosphere of 40° C. and90% RH for 1,000 hours.

Example 22

An electroconductive paste was obtained by following the same process asin Example 17 except that 600 parts by weight (80% by weight) of theflaky silver powder used in Example 17 and 150 parts by weight (20% byweight) of the silver powder having an aspect ratio of 2.3 were blended.The content of the flaky silver powder and the silver powder having anaspect ratio of 2.3 was 45% by volume (87.7% by weight) based on thesolids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed by following the same process as in Example 21 and itscharacteristics were evaluated. As a result, resistivity of the electriccircuit was 10 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 4%. According to theresult of the humidity and DC applied test, the rate of change ofresistivity was 7%.

Example 23

An electroconductive paste was obtained by following the same process asin Example 17 except that 800 parts by weight (72.7% by weight) of theflaky silver powder used in Example 17 and 300 parts by weight (27.3% byweight) of the unsteady-shaped silver powder were blended. The contentof the composite silver powder comprising the flaky silver powder andthe silver powder having an aspect ratio of 2.3 was 54.5% by volume(91.3% by weight) based on the solids of the electroconductive paste. Awiring board was manufactured by following the same process as inExample 17, then an electric circuit was formed thereon and itscharacteristics were evaluated. As a result, resistivity of the electriccircuit was 8.5 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 5%. According to theresult of the humidity and DC applied test, the rate of change ofresistivity was 6%.

Example 24

The wiring board obtained in Example 23 was hot pressed by heated rollsunder the conditions of 125° C. and 980 N/cm at a rate of 30 cm/min andthen cured at 145° C. for 30 minutes to form an electric circuit with adensified circuit pattern. Resistivity of this electric circuit was 8.4μΩ·cm. As a result of the thermal shock test of said wiring board, therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 4%.

Example 25

550 parts by weight (84.6% by weight) of the flaky silver powder used inExample 17 and 100 parts by weight (15.4% by weight) of the Ni powderhaving an aspect ratio of 3 and an average particle diameter of 5 μmwere blended, and this blend was added to 145 parts by weight of theresin composition obtained in Example 1 and uniformly mixed anddispersed by a mixing and grinding machine and a three-roll mill toobtain an electroconductive paste. The content of the flaky silverpowder and the Ni powder was 42.5% by volume (86.6% by weight) based onthe solids of the electroconductive paste. A wiring board was obtainedby following the same process as in Example 17.

Then the above wiring board was hot pressed under the conditions of 110°C. and 10 MPa for 2 minutes and then cured at 145° C. for 30 minutes toform an electric circuit with a densified circuit pattern. Resistivityof this electric circuit was 12 μΩ·cm. As a result of the thermal shocktest, the rate of change of resistivity was 7%. The result of thehumidity and DC applied test of said wiring board gave 6% as the rate ofchange of resistivity. The thermal shock test was conducted by carryingout 100 cycles of heating at 125° C. for 30 minutes and cooling at -65°C. for 30 minutes, and the humidity and DC applied test was conducted bykeeping the test piece in an atmosphere of 40° C. and 90% RH for 1,000hours.

Example 26

An electroconductive paste was obtained by following the same process asin Example 17 except that 650 parts by weight (86.7% by weight) of theflaky silver powder used in Example 17 and 100 parts by weight (13.3% byweight) of the Ni powder were blended. The content of the flaky silverpowder and the Ni powder was 45.5% by volume (88% by weight) based onthe solids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon by following the same process as in Example 25, andits characteristics were evaluated. As a result, resistivity of theelectric circuit was 10 μΩ·cm. As a result of the thermal shock test ofsaid wiring board, the rate of change of resistivity was 4%. Accordingto the result of the humidity and DC applied test, the rate of change ofresistivity was 5%.

Example 27

An electroconductive paste was obtained by following the same process asin Example 17 except that 900 parts by weight (87.4% by weight) of theflaky silver powder used in Example 17 and 130 parts by weight (12.6% byweight) of the Ni powder were blended. The content of the flaky silverpowder and the Ni powder was 53.5% by volume (91% by weight) based onthe solids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon by following the same process as in Example 25, andits characteristics were evaluated. As a result, resistivity of theelectric circuit was 8.2 μΩ·cm. As a result of the thermal shock test ofsaid wiring board, the rate of change of resistivity was 5%. Accordingto the result of the humidity and DC applied test, the rate of change ofresistivity was 5%.

Example 28

The wiring board obtained in Example 27 was hot pressed by heated rollsunder the conditions of 125° C. and 980 N/cm at a rate of 30 cm/min, andthen cured at 145° C. for 30 minutes to form an electric circuit with adensified circuit pattern. Resistivity of this electric circuit was 8.5μΩ·cm. As a result of the thermal shock test of said wiring board, therate of change of resistivity was 5%. According to the result of thehumidity and DC applied test, the rate of change of resistivity was 5%.

Example 29

An electroconductive paste was obtained by following the same process asin Example 17 except that 410 parts by weight (66.7% by weight) of theflaky silver powder used in Example 17 and 205 parts by weight (33.3% byweight) of the Ni powder were blended. The content of the flaky silverpowder and the Ni powder was 41.5% by volume (86.1% by weight) based onthe solids of the electroconductive paste. A wiring board was obtainedby following the same process as in Example 17.

Then the above wiring board was hot pressed under the conditions of 100°C. and 5 MPa for 2 minutes and then cured at 145° C. for 30 minutes toform an electric circuit having a densified circuit pattern. Resistivityof this electric circuit was 12.5 μΩ·cm. As a result of the thermalshock test of said wiring board, the rate of change of resistivity was5%. According to the result of the humidity and DC applied test, therate of change of resistivity was 6%. The thermal shock test wasconducted by carrying out 100 cycles of heating at 125° C. for 30minutes and cooling at -65° C. for 30 minutes, and the humidity and DCapplied test was conducted by keeping the test piece in an atmosphere of40° C. and 90% RH for 1,000 hours.

Example 30

An electroconductive paste was obtained by following the same process asin Example 17 except that 700 parts by weight (87.5% by weight) of theflaky silver powder used in Example 17 and 100 parts by weight (12.5% byweight) of the Ni powder were blended. The content of the flaky silverpowder and the Ni powder was 54.0% by volume (91.1% by weight) based onthe solids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon in the same way as in Example 29 and itscharacteristics were evaluated. As a result, resistivity of the electriccircuit was 9.5 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 4%. According to theresult of the humidity and DC applied test, the rate of change ofresistivity was 5%.

Example 31

An electroconductive paste was obtained by following the same process asin Example 17 except that 750 parts by weight (83.3% by weight) of theflaky silver powder used in Example 17 and 150 parts by weight (16.7% byweight) of the 10 wt % silver plated Ni powder were blended. The contentof the composite silver powder comprising the flaky silver powder andthe silver-plated Ni powder was 50.0% by volume (89.7% by weight) basedon the solids of the electroconductive paste. A wiring board was made byfollowing the same process as in Example 17, then an electric circuitwas formed thereon by following the same process as in Example 29, andits characteristics were evaluated. As a result, resistivity of theelectric circuit was 8.3 μΩ·cm. As a result of the thermal shock test ofsaid wiring board, the rate of change of resistivity was 5%. Accordingto the result of the humidity and DC applied test, the rate of change ofresistivity was 4%.

Example 32

The wiring board obtained in Example 31 was hot pressed by heatedrollers under the conditions of 125° C. and 980 N/cm at a rate of 30cm/min and then cured at 145° C. for 30 minutes to form an electriccircuit having a densified circuit pattern. Resistivity of this electriccircuit was 8.4 μΩ·cm. As a result of the thermal shock test of saidwiring board, the rate of change of resistivity was 4%. According to theresult of the humidity and DC applied test, the rate of change ofresistivity was 4%.

Example 33

A spherical copper powder (SF-Cu produced by Nippon Atomized MetalPowders Corp.) was coated by 25% by weight with silver by a substitutionplating method, and then rolled with zirconia balls in a ball mill at arate of 60 revolutions per minute for 30 minutes to change the shape,thereby obtaining a flaky silver-coated copper powder having an averagemajor particle diameter of 10.3 μm, an aspect ratio of 6 and a copperexposed area of 3-18%, 7% on the average. Separately from the above, thesame type of copper powder as described above was coated by 25% byweight with silver by a substitution plating method, and then rolledwith glass balls in a ball mill at a rate of 60 revolutions per minutefor 20 minutes to change the shape, thereby obtaining an unsteady-shapedsilver-coated copper powder having an average major particle diameter of7.5 μm, an aspect ratio of 2 and a copper exposed area of 2-7%, 3% onthe average. Next, 410 parts by weight (66.7% by weight) of the flakysilver-coated copper powder and 205 parts by weight (33.3% by weight) ofthe unsteady-shaped silver-coated copper powder were added to 145 partsby weight of the resin composition obtained in Example 1 and uniformlymixed and dispersed by a mixing and grinding machine and a three-rollmill to obtain an electroconductive paste. The content of the flakysilver-coated copper powder and the unsteady-shaped silver-coated copperpowder was 86% by weight based on the solids of the electroconductivepaste.

The copper exposed area was determined in the following way. An SEMphotograph of the silver-coated copper powder was taken by a scanningelectron microscope (SEM) and 20 particles of the silver-coated copperpowder were picked up randomly therefrom and subjected to an arealanalysis of the silver and copper particles by an X-ray analyzer,calculating the rate of copper exposure from the areal rates of theportion coated with silver and the portion where copper was exposed. Theaverage of the determinations was shown here as the copper exposed area.The copper exposed area was determined in the same way in the followingExamples and Comparative Examples.

Thereafter, by using the above electroconductive paste, a silverconductor circuit 1 shown in FIG. 1 and FIG. 5 was printed on a 125 μmthick polyethylene terephthalate film, then hot pressed at 80° C. in theatmosphere for 30 minutes and further at 100° C. under a pressure of 5MPa for 2 minutes, and then heat treated at 145° C. for 30 minutes toobtain an electric circuit. The size of A in FIG. 5 was 100 μm.

Resistivity of the obtained electric circuit shown in FIG. 1 was 11.5μΩ·cm. As a result of the thermal shock test of the electric circuit,the rate of change of resistivity was 5%. As a result of the humidityand DC applied test of the interdigital electric circuit shown in FIG.5, the insulation resistance was 10⁸ Ω or more. The thermal shock testwas conducted by carrying out 100 cycles of heating at 125° C. for 30minutes and cooling at -65° C. for 30 minutes, and the humidity and DCapplied test was conducted by applying a voltage of 50 V across theadjoining lines and keeping the test piece in an atmosphere of 40° C.and 90% RH for 2,000 hours.

Example 34

700 parts by weight (87.5% by weight) of the flaky silver-coated copperpowder obtained in Example 33 and 100 parts by weight (12.5% by weight)of the unsteady-shaped silver-coated copper powder obtained in Example33 were added to 145 parts by weight of the resin composition obtainedin Example 1 and uniformly mixed and dispersed by a mixing and grindingmachine and a three-roll mill to obtain an electroconductive paste. Thecontent of the flaky silver-coated copper powder and the unsteady-shapedsilver-coated copper powder was 89% by weight based on the solids of theelectroconductive paste. An electric circuit was made by following thesame process as in Example 33 and its characteristics were evaluated. Asa result, resistivity of the electric circuit was 9.5 μΩ·cm. As a resultof the thermal shock test of the electric circuit, the rate of change ofresistivity was 4%. In the humidity and DC applied test of theinterdigital electric circuit, the insulation resistance was 10⁸ Ω ormore.

Example 35

750 parts by weight (83.3% by weight) of the flaky silver-coated copperpowder obtained in Example 33 and 150 parts by weight (16.7% by weight)of an unsteady-shaped silver-coated copper powder having an averagemajor particle diameter of 6.0 μm, an aspect ratio of 2 and a copperexposed area in the range of 3-13%, averaging 7%, obtained by coatingthe surfaces of the copper powder used in Example 33 with silver by 10%by weight by a substitution plating method and thereafter following thesame process as in Example 33, were added to 145 parts by weight of theresin composition obtained in Example 1 and uniformly mixed anddispersed by a mixing and grinding machine and a three-roll mill toobtain an electroconductive paste. The content of the flakysilver-coated copper powder and the unsteady-shaped silver-coated copperpowder was 89% by weight based on the solids of the electroconductivepaste. An electric circuit was made by following the same process as inExample 33 except that hot pressing was carried out under a pressure of20 MPa, and its characteristics were evaluated. As a result, resistivityof the electric circuit was 8.3 μΩ·cm. As a result of the thermal shocktest of the electric circuit, the rate of change of resistivity was 5%.In the humidity and DC applied test of the interdigital electriccircuit, the insulation resistance was 10⁸ Ω or more.

Example 36

By using the electroconductive paste obtained in Example 35, an electriccircuit was made by following the same process as in Example 33 and thenhot pressed by the heated rolls under the conditions of 100° C. and 10MPa to densify the printed circuit, and its characteristics wereevaluated. As a result, resistivity of the densified electric circuitwas 8.4 μΩ·cm. As a result of the thermal shock test of the densifiedelectric circuit, the rate of change of resistivity was 4%. In thehumidity and DC applied test of the interdigital electric circuit, theinsulation resistance was 10⁸ Ω or more.

Comparative Example 4

400 parts by weight of the flaky silver-coated copper powder obtained inExample 33 was added to 145 parts by weight of the resin compositionobtained in Example 1 and uniformly mixed and dispersed by a mixing andgrinding machine and a three-roll mill to obtain an electroconductivepaste. Then an electric circuit was made by following the same processas in Example 33 except for the hot pressing step, and itscharacteristics were evaluated. As a result, resistivity of the electriccircuit was 62 μΩ·cm. As a result of the thermal shock test of theelectric circuit, the rate of change of resistivity was 10%. In thehumidity and DC applied test of the electric circuit, the insulationresistance was 10⁸ Ω or more.

Comparative Example 5

400 parts by weight of a flaky silver powder having an aspect ratio of 8and an average major particle diameter of 8 μm (trade name TCG-1produced by Tokuriki Chemical Research Co., Ltd.) was added to 145 partsby weight of the resin composition obtained in Example 1 and uniformlymixed and dispersed by a mixing and grinding machine and a three-rollmill to obtain an electroconductive paste. Then an electric circuit wasmade by following the same process as in Example 33 except for the hotpressing step, and its characteristics were evaluated. As a result,resistivity of the electric circuit was 62 μΩ·cm. As a result of thethermal shock test of the electric circuit, the rate of change ofresistivity was 10%. In the humidity and DC applied test of theinterdigital electric circuit, the insulation resistance dropped to 10⁸Ω or less in 370 hours of test time, and there took place the migrationof silver.

Comparative Example 6

To 145 parts by weight of the resin composition obtained in Example 1,40 parts by weight of the copper powder coated with silver on thesurface before being deformed into a flaky shape (the copper exposedarea being less than 1%, almost 0%) obtained in Example 33 was added anduniformly mixed and dispersed by a mixing and grinding machine and athree-roll mill to obtain an electroconductive paste. Then an electriccircuit was made by following the same process as in Example 33 exceptfor the hot pressing step, and its characteristics were evaluated. As aresult, resistivity of the electric circuit was 65 μΩ·cm. As a result ofthe thermal shock test of the electric circuit, the rate of change ofresistivity was 12%. In the humidity and DC applied test of theinterdigital electric circuit, the insulation resistance dropped to 10⁸Ω or less in 530 hours of test time, and there took place the migrationof silver.

Industrial Applicability

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste forforming an electric circuit which is low in resistivity, high inelectroconductivity and minimized in change of resistivity even afterthe thermal shock test and/or the humidity and DC applied test.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste which isespecially excellent in electroconductivity and also in oxidationresistance and heat resistance.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste having ananchoring effect when pressed.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste forforming an electric circuit which is low in resistivity, high inelectroconductivity and also excellent in migration resistance.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste which islow in resistivity, high in electroconductivity, minimized in change ofresistivity even after the thermal shock test and/or the humidity and DCapplied test, and capable of increasing probability of contact betweenthe electroconductive powder particles, elevating electroconductivity ofthe electric circuit, and also raising electroconductivity especiallywhen a circuit is printed on a sheet-like substrate and the printedcircuit is pressed.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste which isespecially excellent in electroconductivity and also in oxidationresistance and heat resistance.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste having ananchoring effect when pressed.

According to the composite electroconductive powder of the presentinvention, it is possible to obtain an electroconductive paste forforming an electric circuit which is low in resistivity, high inelectroconductivity and also excellent in migration resistance.

The electroconductive paste of the present invention is suited forforming an electric circuit which is low in resistivity, high inelectroconductivity and minimized in change of resistivity even afterthe thermal shock test and/or the humidity and DC applied test.

The electroconductive paste of the present invention is suited forforming an electric circuit which is low in resistivity, improved inprobability of contact between the electroconductive powder particles,high in electroconductivity and also excellent in migration resistance.

The electric circuit of the present invention is low in resistivity,high in electroconductivity and excellent in migration resistance.

The electric circuit of the present invention is excellently suited forforming a fine circuit.

According to the electric circuit producing process of the presentinvention, it is possible to produce an electric circuit which is low inresistivity, high in electroconductivity and excellent in migrationresistance.

According to the electric circuit producing process of the presentinvention, it is possible to produce an electric circuit which isexcellently suited for forming a fine circuit.

We claim:
 1. An electroconductive powder mixture comprising anelectroconductive powder having an aspect ratio of 6 or greater and anelectroconductive powder having an aspect ratio of 5 or less, both theelectroconductive powder having an aspect ratio of 6 or greater and theelectroconductive powder having an aspect ratio of 5 or less containingsilver.
 2. An electroconductive powder mixture according to claim 1,wherein the electroconductive powder having an aspect ratio of 6 orgreater is a silver-coated conductor powder having an aspect ratio of 6or greater.
 3. A composite electroconductive powder mixture according toclaim 2, wherein the silver-coated conductor powder having an aspectratio of 6 or greater is a silver-coated copper powder or asilver-coated copper alloy powder.
 4. An electroconductive powdermixture according to claim 3, wherein the silver-coated conductor powderhaving an aspect ratio of 6 or greater is a silver-coated copper powderor a silver-coated copper alloy powder in which the copper or copperalloy powder is partly exposed.
 5. An electroconductive powder mixtureaccording to claim 1, wherein the electroconductive powder having anaspect ratio of 5 or less is made of a conductor having a higherhardness than silver or silver alloys and coated with silver.
 6. Anelectroconductive powder mixture according to claim 5, wherein theconductor having a higher hardness than silver or silver alloys is apowder of Co, Ni, Cr, Cu, W or an alloy thereof.
 7. An electroconductivepowder mixture according to claim 5, wherein the conductor having ahigher hardness than silver or silver alloys is a copper powder or acopper alloy powder.
 8. An electroconductive powder mixture according toclaim 5, wherein the electroconductive powder having an aspect ratio of5 or less, coated with silver, is a silver-coated copper powder in whichthe copper powder is partly exposed.
 9. An electroconductive powdermixture according to claim 1, wherein the electroconductive powderhaving an aspect ratio of 6 or greater is contained in an amount of 95to 50% by weight, and the electroconductive powder having an aspectratio of 5 or less is contained in an amount of 5 to 50% by weight, atotal being 100% by weight.
 10. An electroconductive powder mixtureaccording to claim 1 consisting essentially of the electroconductivepowder having an aspect ratio of 6 or greater and the electroconductivepowder having an aspect ratio of 5 or less.
 11. An electroconductivepowder mixture according to claim 10, consisting of theelectroconductive powder having an aspect ratio of 6 or greater and theelectroconductive powder having an aspect ratio of 5 or less.
 12. Anelectroconductive powder mixture according to claim 1, wherein anaverage particle diameter of the electroconductive powder having anaspect ratio of 6 or greater is at most 25 μm, and an average particlediameter of the electroconductive powder having an aspect ratio of 5 orless is in a range of 3-20 μm.
 13. An electroconductive powder mixtureaccording to claim 1, wherein the silver is part of a silver alloy, thesilver alloy being an alloy with palladium or platinum.
 14. Anelectroconductive paste comprising the electroconductive powder mixtureof claim 12, a binder and a solvent.
 15. An electroconductive pasteaccording to claim 14, wherein the electroconductive powder mixture iscontained in an amount of 85-93% by weight based on the solids of theelectroconductive paste.
 16. An electroconductive paste according toclaim 15, wherein said electroconductive powder mixture comprises theelectroconductive powder having an aspect ratio of 5 or less an amountof 5-50% by weight and the electroconductive powder having a aspectratio of 6 or greater in an amount of 95-50% by weight, a total being100% by weight.
 17. An electric circuit formed on a surface of asubstrate by applying the electroconductive paste of claim 14 to thesubstrate.
 18. An electric circuit according to claim 17, whereinresistivity of the electric circuit formed on the surface of a substrateis 25 μΩ·cm or less.
 19. An electric circuit formed on a surface of asubstrate by applying the electroconductive paste of claim 15 thesubstrate.
 20. An electric circuit according to claim 19, wherein theelectric circuit has a resistivity of 25 μΩ·cm or less.
 21. An electriccircuit formed on a surface of a substrate by applying theelectroconductive paste of claim 16 to the substrate.
 22. An electriccircuit according to claim 21, wherein the electric circuit has aresistivity of 25 μΩ·cm or less.
 23. A process for producing an electriccircuit, which comprises forming a circuit pattern on a surface of asubstrate with an electroconductive paste of claim 14, and then pressingand curing the paste.
 24. A process for producing an electric circuitaccording to claim 23, wherein the electric circuit formed on thesubstrate surface has resistivity of 25 μΩ·cm or less.
 25. A process forproducing an electric circuit, which comprises forming a circuit patternon a surface of a substrate with the electroconductive paste of claim15, and then pressing and curing the paste.
 26. A process according toclaim 25, wherein the electric circuit has a resistivity of 25 μΩ·cm orless.
 27. A process for producing an electric circuit, which comprisesforming a circuit pattern on a surface of a substrate with theelectroconductive paste of claim 16, and then pressing and curing thepaste.
 28. A process according to claim 27, wherein the electric circuithas a resistivity of 25 μΩ·cm or less.